The present invention relates generally to a hydrostatic self-aligning loading mechanism for applying a controlled axial and/or torsional load to a test specimen without introducing bending moments induced by conventional testing means.
Recent developments in carbon-carbon composites and ceramics have substantially advanced the state of the art and have resulted in the development of interest in such materials for use in structural applications.
Currently, there is no way to test these materials in a multiaxial stress state without imparting some undesirable stresses to the materials being tested due to alignment problems in the apparatus loading the test specimen.
Considerable effort has been made in an attempt to satisfy a long-felt need for determining properties of brittle materials such as carbon-carbon composites and ceramics. Most efforts have focused on compressive, torsional, and tensile tests. A need exists for expanding capabilities to include multiaxial testing. Because brittle materials are extremely sensitive to misalignment during testing, it has not been possible to test these types of materails biaxially, that is, in a combined axial-torsional mode. These types of tests are imperative, however, for developing design guidelines for structural applications of brittle materials. Accordingly, a means is needed that will maintain alignment of a brittle test specimen continually during a test involving combined axial-torsional loading.
One device which provides an alignment function is a universal joint. However, a universal joint has too much play in the bearings and imposes significant bending moments on the specimen. If the bearings are replaced by flexures, the universal joint does not have the necessary load carrying capacity.
U.S. Pat. No. 4,686,860 describes a device which allows only axial testing. In this invention a multiplicity of hydraulic piston assemblies are equally spaced in a hydraulic housing assembly on a circle about the centerline of the tensile specimen; the hydraulic piston assemblies being interconnected by oil channels that are a part of an oil distribution manifold. A pull rod assembly is provided with a circular flange which rests on the top of the piston assemblies and a pull rod which extends downwardly through the center hold of the housing assembly for gripping the pull rod end of a ceramic specimen.
Use of a hydraulic fluid as a distribution medium to divide the applied load into a multiplicity of equal parts counteracted by the circular array of miniature pistons is a key feature of this invention. The seals used in this device cause some amount of friction which imposes an undesirable bending movement in a specimen.
It is accordingly a general object of this invention to provide a means for accurately testing the tensile strength of brittle specimens.
it is also an object of this invention to provide a means for aligning ceramic grippers.
Another more particular object of the invention is to provide a means for applying biaxial stress to a specimen without inducing significant bending moments in the specimen.